Constructors in Java

In Java, a constructor is a block of codes similar to the method. It is called when an instance of the class is created. 

Rules for creating Java constructor

There are two rules defined for the constructor.

1. Constructor name must be the same as its class name
2. A Constructor must have no explicit return type
3. A Java constructor cannot be abstract, static, final, and synchronized

Note: We can use access modifiers while declaring a constructor. It controls the object creation. In other words, we can have private, protected, public or default constructor in Java.

Types of Java constructors

There are two types of constructors in Java:

1. Default constructor (no-arg constructor)
2. Parameterized constructor

Java Default Constructor

A constructor is called "Default Constructor" when it doesn't have any parameter.

example

1. //Java Program to create and call a default constructor  
2. class Bike1{  
3. //creating a default constructor  
4. Bike1(){System.out.println("Bike is created");}  
5. //main method  
6. public static void main(String args[]){  
7. //calling a default constructor  
8. Bike1 b=new Bike1();  
9. }  
10. }  

O/P : Bike is created

Rule: If there is no constructor in a class, compiler automatically creates a default constructor.

Q) What is the purpose of a default constructor?

The default constructor is used to provide the default values to the object like 0, null, etc., depending on the type.

Example of default constructor that displays the default values

1. //Let us see another example of default constructor  
2. //which displays the default values  
3. class Student3{  
4. int id;  
5. String name;  
6. //method to display the value of id and name  
7. void display(){System.out.println(id+" "+name);}  
8.   
9. public static void main(String args[]){  
10. //creating objects  
11. Student3 s1=new Student3();  
12. Student3 s2=new Student3();  
13. //displaying values of the object  
14. s1.display();  
15. s2.display();  
16. }  
17. }  

O/P : 

0 null
0 null

Explanation:In the above class,you are not creating any constructor so compiler provides you a default constructor. Here 0 and null values are provided by default constructor.

Java Parameterized Constructor


A constructor which has a specific number of parameters is called a parameterized constructor.





Why use the parameterized constructor?

The parameterized constructor is used to provide different values to distinct objects. However, you can provide the same values also.



example



//Java Program to demonstrate the use of the parameterized constructor.  
class Student4{  
    int id;  
    String name;  
    //creating a parameterized constructor  
    Student4(int i,String n){  
    id = i;  
    name = n;  
    }  
    //method to display the values  
    void display(){System.out.println(id+" "+name);}  
   
    public static void main(String args[]){  
    //creating objects and passing values  
    Student4 s1 = new Student4(111,"Karan");  
    Student4 s2 = new Student4(222,"Aryan");  
    //calling method to display the values of object  
    s1.display();  
    s2.display();  
   }  
}  


O/P


111 Karan
222 Aryan


Constructor Overloading in Java

In Java, a constructor is just like a method but without return type. It can also be overloaded like Java methods.





Example of Constructor Overloading



//Java program to overload constructors  
class Student5{  
    int id;  
    String name;  
    int age;  
    //creating two arg constructor  
    Student5(int i,String n){  
    id = i;  
    name = n;  
    }  
    //creating three arg constructor  
    Student5(int i,String n,int a){  
    id = i;  
    name = n;  
    age=a;  
    }  
    void display(){System.out.println(id+" "+name+" "+age);}  
   
    public static void main(String args[]){  
    Student5 s1 = new Student5(111,"Karan");  
    Student5 s2 = new Student5(222,"Aryan",25);  
    s1.display();  
    s2.display();  
   }  
}  






O/P

111 Karan 0
222 Aryan 25



Difference between constructor and method in Java

There are many differences between constructors and methods. They are given below.

Java Constructor
Java Method
A constructor is used to initialize the state of an object.
A method is used to expose the behavior of an object.
A constructor must not have a return type.
A method must have a return type.
The constructor is invoked implicitly.
The method is invoked explicitly.
The Java compiler provides a default constructor if you don't have any constructor in a class.
The method is not provided by the compiler in any case.
The constructor name must be same as the class name.
The method name may or may not be same as the class name.